1. Field of the Invention
The present invention relates to a speech coding system for compression of data of speech signals, and more particularly relates to a speech coding system using analysis-by-synthesis (A-b-S) type vector quantization for coding at a transmission speed of 4 to 16 kbps, that is, using vector quantization performing analysis by synthesis.
2. Background of the Related Art
Speech coders using A-b-S type vector quantization, for example, code-excited linear prediction (CELP) coders, have in recent years been considered promising as speech coders for compression of speech signals while maintaining quality in intracompany systems, digital mobile radio communication, etc. In such a quantized speech coder (hereinafter simply referred to as a "coder"), predictive weighting is applied to the code vectors of a codebook to produce reproduced signals, the error powers between the reproduced signals and the input speech signal are evaluated, and the number (index) of the code vector giving the smallest error is decided on or determined and sent to the receiver side.
A coder using the above-mentioned A-b-S type vector quantization system performs processing so as to apply linear prediction analysis filter processing to each of the vectors of the sound generator signals, of which there are about 1000 patterns stored in the codebook, and retrieve from among the approximately 1000 patterns the one pattern giving the smallest error between the reproduced speech signals and the input speech signal to be coded.
Due to the need for instantaneousness in conversation, the above-mentioned retrieval processing must be performed in real time. This being so, the retrieval processing must be performed continuously during the conversation at short time intervals of 5 ms, for example.
As mentioned later, however, the retrieval processing includes complicated computation operations of filter computation and correlation computation. The amount of computation required for these computation operations is huge, being, for example, several 100M multiplications and additions per second. To deal with this computational complexity, even with digital signal processors (DSP), which are the highest in speed at present, several DSP chips are required. In the case of use for cellular telephones, for example, there is the problem of achieving a small size and a low power consumption.